I am running a standard Amal 389 and jetted as per the instruction book on my Cruiser 80 AMC 25T. My bike is 1958, but i'm aware that the 1957 model was fitted with a main jet of 420, and all subsequent years went to a main jet of 320. Why such a change? I cruise at a around 55mph on a flat, that's with a 17T gearbox sprocket. I will fit an 18T when I can find a decent one.Also, has anyone tried the Mikuni carb route? I'm considering on fitting a Mikuni carb as I know that they are of a superior jetting and quality (I once fitted a Mikuni carb on my old Bantam and it made a world of difference) so any views or experience would be much appreciated.

I am not entirely sure of the reasons behind the carb change, but initial tests of the 250cc AMC engine showed a tendency to run hot, and AMC's answer was to artificially cool it by passing large volumes of fuel into the cylinder, unfortunately not all of this was burnt and made for an oily/wet exhaust and high fuel consumption, for little gain over the Viliiers 197cc engine.Also there was a problem with lubrication to the main bearings on the initial engines that showed as early failures and warranty claims.Presumably feedback from use by customers and tests by Amal necessitated the change, engines that were first modified to overcome the bearing lubrication problems were pre-fixed with the letter M i.e. M25TThe Piatti designed loop scavenge system seems to require a lot more fuel for it to work than the standard piston port design.I have had several machines ( all competition) where Villiers 250cc piston, head and barrel have been fitted onto the AMC crankcases with an adaptor plate which is about 3/4" thick, but easy to work out as whatever the AMC barrell length is the Villiers will need to be set up to be the same length , hence the need for a spacer as the Villliers cylinder is shorter,Both machines use almost identical bore and stroke 66mm bore x 72mm stroke, so as long as the overall length from the crankcase to the cylinder top is the same, all porting will be correct .JH

I thought the huge jet was something to do with over heating of the cylinder. About a month ago I carried out a top end de-carbon and was very pleased with the results. I had covered approximately 5000 miles last year (2015) on a new piston, rings and rebore. Upon checking the ring clearance, I found that they had only worn 3thou so well within the tolerances, in fact on these figures they should be good for another 15,000 miles. Whilst carrying out the maintenance it was clear that the crank was getting well lubricated, the oil was nice and clear and had provided a good coating.Going back to the carb question, are Mikuni in your opinion a lost leader for these bikes?

Christian , I have no experience of them , but as they are a more modern design it is unlikely they will be any worse ? getting the set up right , might be another thing and may sacrifice a few pistons along the way ?JH

I assume the 389 is a Monoblock carb? That is a big jump of main jet but as John said excessive fuel saps heat from its surroundings as it evaporates . I am not familiar with the Mikuni but hear good reports but find it hard to go non English with parts. Why not try a concentric they are simple. Remember the main jet only works with the throttle 3/4 to Full open.

Ken , no the studs do not align , that is why I said an adaptor plate is needed, Allen bots through plate to AMC stud holes, then studs up from plate on different pattern to suit the Villiers barrel. The adaptor plate has to be the thickness of the difference in length between the AMC ( longer) barrel and the Villiers ( shorter) barrel, the piston will then reach the top of the Villiers cylinder, all ports will be in the correct place as the stroke is the same on both engines (72mm)#It could probably done on an AMC 20T as well as the stroke and bore match the Villiers 197cc, but have never seen it done ? only seen it on the 250cc 25T engineJH

I read a post from another Cruiser 80 owner who stated that he rode all day at 60mph+ without any issues. I seem to get to 55mph and then the throttle tops out. I'm not looking to go faster for any other reason than I just find it interesting to eke a little more out of the ole girl. I had thought about going down the Concentric route but I doubt very much that it would be a marked improvement. I will contact Amal on Monday next week to gain their views. I think that i'm probably most likely to try the Mikuni carb just out of curiosity. I will keep you posted!

The Monoblock is a good carb. One important thng that goes out of adjustment if float height. On the chamber cover in the word AMAL is. A Dot. That should be the fuel height Ihave a drilled cover somewhere so you can set it . Another thing is the needle jet wear. Have you checked all jets numbers and the needle. And even carb type numbers Ken

Ken ,another lessonMono bloc carbs have a code 389 is the body style/size 27 is the specification for a particular manufacturer There will be loads of 389 carbs numbered from 389/1 to wherever it finished some where in the 200's i.e. 389/213389 carbs ranged in size from 1 3/32" to 1 3/16"376 Carrbs ranged in sizes up to 1 1/16"375 carbs ranged up to 15/32"

The 27 has nothing to do with size, it is just the bore size , jets, slide, jet block that as a package goes to make up specification 27 for Fb Cruiser 80JH

My mistake John. On the Concentric it does mean the carb bore.. Ie 626/17 600 range 26mm bore 17 jetted for a bantam. If you look at the Amal web site it shows many different carbs and settings for the Cruiser 80 Should be 1 1/8" bore

Yes Ken, the metric bore size applies to Amal concentrics and later smooth bores, but not anything pre 1966, so kind of rules FB's out as they never fitted concentrics.on the Villiers powered trials bikes FB used a pre monobloc carb on these you have a body code i.e 276 then a manufacturers coding i.e. FYthen a float bowl code i.e. 2A Hence 276FY/2A would be a 276 Carbuettor specification FY for a Francis Barnett 197cc trials bike with a 2A top feed float bowl correct spec for a 1953 falcon trialsYes agree with the info you give, I have all of the Amal catalogues from 1934 to 1970'sstrangely enough Villiers were doing the metric size thing way before Amal from about 1954S19 =19mms25 =25mm S22 = 22mm

Ken, early motors used in the UK were generally of Continental design , De Dion Bouton, Minerva, MAG etc even the founders of Triumph were German and initially used Minerva engines !so I guess we got into the habitNearly all pistons were measured in mm , noticeable exceptions were Matchless and JAP and early capacities were measured in the old Horse Power rather than CC, but somewhere along the line I guess we were closer to Europe than USA that continued with Cubic Inches ?? so we fell in with our near neighbours who recognised the CC readily as they had adopted the metric system years before ??Just a guess ??JH

Hi,happy new year to all,I was under the impression that the reduction in main jet size was due to the modification of the main jet block for two strokes by amal.Later carbs have an angled spray tube,earlier ones being flat topped as on a four stroke.If you have a carb with the angled spray tube it should have the smaller main jet .Hope this helps.

I have read this topic with interest as I am currently struggling with my recently acquired 1962 Cruiser 80 which is fitted with an AMC M25S (ie scrambler) engine. It is almost impossible to start and even when it does very occasionally catch it will only run (quite well) for maybe 10 seconds before it dies. There is a consistent spark, and I have checked and re-checked the timing and tried variations with advancing and retarding. The bike consistently kicks back against a gentle turn over on the kickstart when easing past TDC.

My presumption is that it is either a carb problem, or something more fundamental such as condenser/ coil, bore/ ring problems or dried out seals. As the bike can run (albeit not very often or for long) I think that the issue is very likely carb related, and float height is my prime suspect. Before I am forced to buy new ignition components or strip the engine, I want to rule out the carb. The carb and settings are as standard for a scrambler ie 389/28 with a 290 main jet. I think that the float height might be the culprit but until I can make a perpex window or rig up a clear tube to the pilot jet am not sure.

Note that as this is the scrambler engine the comp ratio is quite high and the ignition components may be different to standard (eg points cam?). It is certainly doing my duff right hip no good at all. Any knowledgeable thoughts based on relevant experience?

whippet jim wrote:Hi,happy new year to all,I was under the impression that the reduction in main jet size was due to the modification of the main jet block for two strokes by amal.Later carbs have an angled spray tube,earlier ones being flat topped as on a four stroke.If you have a carb with the angled spray tube it should have the smaller main jet .Hope this helps.

regards,Jim.

If that's the case Jim that will make a big difference. I am converting a members bantam that had been fitted with the 4 Stroke carb and that was one of the differences. With that altered it alters everything else.

whippet jim wrote:Hi,happy new year to all,I was under the impression that the reduction in main jet size was due to the modification of the main jet block for two strokes by amal.Later carbs have an angled spray tube,earlier ones being flat topped as on a four stroke.If you have a carb with the angled spray tube it should have the smaller main jet .Hope this helps.

regards,Jim.

If that's the case Jim that will make a big difference. I am converting a members bantam that had been fitted with the 4 Stroke carb and that was one of the differences. With that altered it alters everything else.

If it is kicking back when eased over top dead centre it suggests timing is close, if not maybe a little advanced ? If fuel is paced down the plughole does it start and run until the fuel is used up ? If so this would point towards a carb problem ? I presume you are tickling / flooding the carb before attempting to start from cold and have the choke slide in the closed ( down ) position? Try keeping your finger on the tickler when it starts to keep the mixture rich until the engine warms up . On trials Bultacos in the past I have had to lean the bike over at 45 degrees with tickle button at the higher side and keep my finger on the tickler once started until the motor was running smooth, and on cold days it was necessary to leave the back wheel in the van, with the front on the ground and kick them in this position so that fuel was literally running into the bore, once warm it was no problem !JH

piston 197 wrote:If it is kicking back when eased over top dead centre it suggests timing is close, if not maybe a little advanced ? If fuel is paced down the plughole does it start and run until the fuel is used up ? If so this would point towards a carb problem ? I presume you are tickling / flooding the carb before attempting to start from cold and have the choke slide in the closed ( down ) position? Try keeping your finger on the tickler when it starts to keep the mixture rich until the engine warms up . On trials Bultacos in the past I have had to lean the bike over at 45 degrees with tickle button at the higher side and keep my finger on the tickler once started until the motor was running smooth, and on cold days it was necessary to leave the back wheel in the van, with the front on the ground and kick them in this position so that fuel was literally running into the bore, once warm it was no problem !JH

Yes John you would think that but I have tried slightly advancing and retarding the ignition without any appreciable difference. Notwithstanding I need to re-check the timing when I have more light and time, but I do find it difficult to set as the points cam is pretty much like an on/off switch ie with a very steep cam, so a tiny degree of movement and the points are either fully closed or fully open.